New soft magnetic materials will be vital for the next generation of power electronics for a broad set of applications including the electrical grid, transportation, and defense applications. Additionally, new hard (permanent) magnetic materials will be required in the next generation of electrical motors, generators, actuators, and other devices. Current state-of-the-art soft magnetic materials do not meet the needs of power electronics, particularly next generation systems envisioned to operate at high frequencies (>1 kHz). Additionally, the best performing soft magnetic materials are costly to manufacture. The best performing permanent magnets contain significant quantities of rare earth elements. Because these minerals are expensive and in limited supply, alternative materials must be developed to replace rare earth based magnets in motors, generators, and actuators. Iron nitrides (e.g. γ′-Fe4N, α″-Fe16N2, and FeN), are comprised entirely of low-cost and abundant elements, and will enable better performing soft and hard magnetic materials. See S. Bhattacharyya, J. Phys. Chem. C 119, 1601 (2015).
Iron nitrides have been known and studied for many decades due to their impressive mechanical and magnetic properties. See A. Fry, Stahl Eisen 43, 12 (1923); K. H. Jack, Proc. R. Soc. A A208, 200 (1951); T. K. Kim and M. Takahashi, Appl. Phys. Lett. 20, 492 (1972); S. Okamoto et al., J. Appl. Phys. 85, 4952 (1999); A. Leineweber et al., Phys. B 276/278, 266 (2000); and M. Tayal et al., Surface and Coatings Technology 275, 264 (2015). According to experimental results from thin films and theoretical calculations, iron nitrides should have magnetic moments well in excess of current state of the art magnetic materials. See Y. Takagi et al., Phys. Rev. B 81, 035422 (2010); S. Bhattacharyya, J. Phys. Chem. C 119, 1601 (2015); and Z. N. Kayani et al., Surface Review and Letters 21, 1450013 (2014).
Therefore, γ′-Fe4N (a soft magnet) would be ideally suited for use in applications such as transformer and inductor cores. Conversely, α″-Fe16N2 and FeN could serve as replacements for current state-of-the-art permanent magnets. Most iron nitrides have only been fabricated as thin films, powder, or inclusions in other materials. See Z. N. Kayani et al., Surface Review and Letters 21, 1450013 (2014); and P. Prieto et al., Surface and Interface Analysis 38, 392 (2006). Bulk iron nitrides have rarely been fabricated because a high sintering temperature is required to fully consolidate these materials using conventional sintering processes. In particular, the decomposition of iron nitrides on heating to approximately 670° C. is problematic. See S. Ito, J. Jpn Soc. Powder Metall. 43, 1415 (1996).
Therefore, a need remains for a viable, low cost, and scalable method to synthesize bulk iron nitrides.